The involvement of the autonomic nervous system in the etiology and/or maintainance of essential hypertension has been proposed. Recent evidence has shown that some patients with this disorder respond well to treatment with beta adrenergic blocking agents. Other recent studies have shown that adrenergic receptor sensitivity changes with temperature, and it has been proposed that this change represents allosteric changes (i.e. alpha to beta) in a single adrenergic receptor. Thus, the objectives of this study are to determine whether adrenergic receptor sensitivity changes in vivo as it does in vitro and to determine whether these changes might be involved in essential hypertension. Data from these studies will also aid in the understanding of the mechanism of action of beta blocking agents in treatment of hypertension. Cardiovascular responses of DOCA-hypertensive and spontaneously hypertensive rats to adrenergic agonists and antagonists at various temperatures will be compared to responses of appropriate control animals. Blood pressure, heart rate and hind limb flow will be monitored. A series of in vitro experiments will be performed using aortic strips and left ventricular strips from hypertensive and control animals. Dose response relationships will also be determined in this series of experiments to adrenergic agonists and antagonists at various temperatures.